As used herein, the term "geosynthetic material" refers to and connotes, but is not limited to, such materials as woven, braided, or other formed synthetic textile materials, plastics, fiberglass, resinous materials, woven synthetic cloth, matting, webbing, sheeting, composites and mixtures of those materials, and other such materials.
In the design, development and manufacture of geosynthetic materials, it is often necessary to determine the stress resistant characteristics of the material including the material tensile strength.
Prior to the subject invention, a number of devices were available for testing material tensile strength, which devices included various types of grip device for securing a piece in the testing device during the test procedure. Those prior art grip devices provide synchronous activation to engage or release opposing grip members. In the prior art, various means are described to provide replaceable grip members and to provide controlled gripping force to the test piece. Other prior art devices include threaded drive means for engagement and release of opposing grip members. Examples of such structures in the prior art are described in the patents and publications described below.
U.S. Pat. No. 2,600,923 describes a grip device which has manually actuated opposing grip jaws that are engaged or disengaged by operation of a lever mounted on an external surface of the grip device and connected to the separate jaws by various mechanical linkages. The amount of grip force applied to the test piece is controlled and preselected by the operator setting a pin in one of a plurality of registered pairs of force selection holes formed in contiguous members of the grip apparatus parts. The grip jaws of that device are replaceable and mounted on opposing interior faces of grip jaw support members by threaded fasteners, to which access is obtained by partial disassembly of the apparatus. That prior art device permits only incremental variation in the grip force applied to the work piece and does not permit continuous variation in the amount of grip force applied. In addition, removal and replacement of the grip jaws is cumbersome and time consuming.
U.S. Pat. No. 3,247,565 describes grip apparatus in which retaining force is applied to the test piece in a direction transverse to the direction of application of the test force or tensile pull. That apparatus includes one grip member fixed and secured in the body of the device and spaced from an opposing, movable grip member, adjustable by turning two adjusting bolts connected to the movable grip member and rotatably mounted in the frame of the device. That grip apparatus requires manual operation of the adjusting bolts and applies the grip force, not over the full area of the test piece, but rather over limited areas or lines of force on the test piece. In addition, that device does not provide means for overcoming those problems peculiar to gripping tensile stressed geotextile materials including, at least, the necessity to avoid substantial deformation, tearing, slipping, cutting, creep and movement of the test piece.
U.S. Pat. No. 2,350,060 describes a grip device for materials subjected to compression testing, in which the gripping forces are applied along a plurality of tangent lines between grip rollers and the surface of the test piece, with the grip force applied in a direction that is transverse to the direction of the compressive force applied to the test piece. One of two grip blocks remains stationary, with the opposing grip block moved toward the stationary grip block by manual actuation of a pair of adjustment screws connected to the movable block and threadably and rotatably mounted in the frame of the device. That device does not provide means to resolve the problems peculiar to gripping geosynthetic materials during tensile stress testing, including prevention of slip, cutting, tearing, creep and other damage to the test piece.
U.S. Pat. No. 3,461,719 describes a viscoelastometer grip apparatus which addresses problems of creep or slippage of test pieces made of plastic or synthetic resinous materials, especially when tested in high temperature and/or gaseous environments. That grip apparatus includes a pair of opposing grip jaw assemblies, each including oblique walls acting as guide surfaces for contained wedge shaped grip jaws, which apply grip forces in directions oblique to the direction of application of the tensile pull. Easier mounting of the test piece in the opposing grip clamps is enabled by first mounting the opposing grip clamps on a jig which permits one grip clamp to be set in fixed position while the opposing grip clamp is adjusted by movement through a slotted aperture in the jig to permit proper setting of the test and imposition of the initial grip forces to the test piece. Thereafter, the opposing clamps are released from the jig assembly and mounted on the tensile test device for application of the tensile stress to the test piece. Although that device does provide means for mounting and loading test pieces in high temperature, gaseous test environments, that patent does not disclose means for preventing creep and slippage and overcoming test piece damage and deformation in tensile stress testing of geosynthetic materials.
U.S. Pat. No. 2,419,711 describes a tensile test grip device having grip jaw mounting guides within inclined planar surfaces which drive opposing serrated grip jaw pieces toward each other into contact with the peripheral surfaces of a test piece to apply grip forces obliquely to the direction of test force applied to the test piece. That device describes a clamp band surrounding the grip jaw holders and having a threaded set screw extending through the clamp and engaging an outer surface of one of the grip jaw holders to move the grip jaw holders toward each other and to apply the initial, starting grip on the test piece before the test force is applied. That apparatus does not address the particular needs of grip apparatus for tensile testing of geosynthetic materials.
It is an object of this invention to provide grip apparatus for tensile stress testing devices for geosynthetic materials, which grip apparatus is strong and relatively light weight and uses a variety of different size grip jaws which are easily inserted into, removed from or replaced in the grip jaw support members of the apparatus without requiring substantial disassembly of the apparatus.
It is a further object of this invention to provide such grip apparatus for geosynthetic materials to substantially reduce damage to the test piece by providing opposing grip jaw structures which can be actuated and moved selectively either in mutually synchronous relationship or in asynchronous relationship to provide engagement with a test piece.
Another object of this invention is to provide grip apparatus for geosynthetic materials in which at least one of the grip jaws can be selectively preset in a stationary position and the opposing grip jaw can be carefully actuated by controlled application of relatively small force increments systematically applied to the movable grip member to assure effective gripping of the test piece while avoiding creep, slippage, deformation or damage to the test piece.
Another object of this invention is to provide grip apparatus for stress testing of geosynthetic materials wherein the grip members are actuated by fluids in a controlled manner to apply either continuous or relatively small increments of actuating force selectively at separate points on the test piece.